Method of operating a combustion apparatus

ABSTRACT

A method of operating a combustion apparatus such as an internal combustion engine ( 10 ) is described, in which the apparatus ( 10 ) includes at least one combustion chamber ( 11 ) with an inlet port ( 15 ) for primary combustion air, means ( 19 ) to introduce into the combustion chamber ( 11 ) primary fuel for combustion with the primary air, an exhaust port ( 16 ) for combustion products, and an exhaust system ( 24 ) for exhausting the combustion products to atmosphere, the method including introducing into the exhaust system ( 24 ) secondary air, mechanically acting upon the secondary air and products of combustion in the exhaust system ( 24 ) in the presence of a catalyst, to produce a reformed fuel, introducing the reformed fuel into the combustion chamber ( 11 ) for combustion with primary fuel and primary air.

[0001] This invention relates to a method of operating a combustionapparatus and more particularly but not exclusively an internalcombustion engine e.g. for a vehicle. Such an engine typically includesat least one combustion chamber with an inlet port for primarycombustion air, means to introduce into the combustion chamber primaryfuel for combustion with the primary air, an outlet port for combustionproducts, and an exhaust system for exhausting the combustion productsto atmosphere. The primary fuel may be petrol, diesel, liquid petroleumgas, for example, or any other suitable fuel or mixture of such fuels.

[0002] The development of internal combustion engines has tended tofocus on maximising the power output of the engine, achieving economy offuel use, and the reduction of harmful emissions.

[0003] Particularly in the case of achieving reduction of harmfulemissions, development has tended to concentrate on improving combustionwithin the engine for example by developing fuel injection systems whichprovide for improved fuel/air mixing in combustion chambers of theengine, and then cleaning combustion products, for example by passingthe combustion products through catalytic converters for example.

[0004] The provision of catalytic converters is problematic because suchdevices only tend to operate to their maximum performance once very hot,such that during short journeys for example in which engines may notattain an optimum operating temperature, such catalytic convertersprovide substantially no beneficial effect. Moreover, such convertersare expensive and require frequent replacement, as they are easilycontaminated.

[0005] In previous patent application EP-A-0744006 there is proposed anapparatus for improving combustion, by introducing into the combustionchamber of the engine, a vaporised secondary hydrocarbon based highcalorific value fuel, obtained by heating a mineral oil. Thus theefficiency with which the primary fuel is burned in the engine isenhanced, and this has the effect of reducing the production of harmfulexhaust emissions.

[0006] In previous patent application EP-A-041831 there is described anexhaust aspirator which is adapted to permit the introduction ofrelatively clean air into an exhaust manifold of the engine to promotemore effective exhaust gas discharge and to cause aspirated air to bedrawn into a combustion chamber of the engine through an exhaust portthereof, rather than exhaust gases, during cyclic pressure changes whichoccur in normal engine operation.

[0007] According to a first aspect of the invention we provide a methodof operating a combustion apparatus which includes at least onecombustion chamber with an inlet port for primary combustion air, meansto introduce into the combustion chamber primary fuel for combustionwith the primary air, an exhaust port for combustion products, and anexhaust system for exhausting the combustion products to atmosphere, themethod including introducing into the exhaust system secondary air,mechanically acting upon the secondary air and products of combustion inthe exhaust system in the presence of a catalyst, to produce a reformedfuel, introducing the reformed fuel into the combustion chamber forcombustion with primary fuel and primary air.

[0008] Preferably, the reformed fuel is introduced into the combustionchamber via the exhaust port, e.g. soon after production, but it isenvisaged that the reformed fuel may be collected and stored, for lateruse, and may be fed with the primary fuel into the combustion chamber.

[0009] It has been found that by utilising the method of the invention,a substantial improvement in combustion efficiency can be achieved,resulting in much cleaner exhaust emissions and a significantimprovement in power output. Also whereas with previous proposals forimproved e.g. engine designs, it has been necessary to compromisebetween fuel efficiency and power output, utilising the presentinvention, such compromise is unnecessary.

[0010] Where the invention is applied to an engine, the method mayinclude introducing the secondary air into the exhaust system via anexhaust aspirator e.g. similar to that disclosed in EP-A-0041831, whichdraws air into the exhaust system during low pressure or partial vacuumconditions occurring during the cycle of pressure changes which occur inthe exhaust system during normal operation of the engine. Howeverwhereas in EP-A-0041831, the primary function of the aspirator is todamp down vacuum waves occurring in the exhaust system to improve theefficiency of exhaust gas discharge, in the method of the invention theexhaust aspirator is tuned to draw air into the exhaust system andmechanically to act upon the secondary air and products of combustion bymeans of pressure pulses, in a manner to optimise reformed fuelproduction.

[0011] Typically, an aspirator means suitable for the purpose of themethod of the invention, includes a valve with a spring biased valvemember, the force of the spring acting to urge the valve member intoengagement with a valve seat to close the valve, and the force of thespring being overcome when sufficiently low pressure is developed in theexhaust system, to allow the secondary air to be drawn in through thevalve. Tuning of such an aspirator means for the performance of theinvention may involve adjusting the spring pressure so that the valveresponds to a desired low pressure or partial vacuum at an appropriatetime during the engine cycle, and in a manner to produce strong pressurepulses in the exhaust system adequate mechanically to act upon thecombustion products and secondary air present in the exhaust system toproduce reformed fuel.

[0012] It will be appreciated that in the case of an engine having asingle combustion chamber, the spring may be adjusted to open the valveand allow the secondary air to be drawn into the exhaust system atcommencement of the piston's induction stroke, when a low pressure isdeveloped in the exhaust system as a result of the (single) pistonmoving in the combustion chamber to induce air therein through an airinlet port, prior to the exhaust port being completely closed by anexhaust outlet valve or the piston. In a multi-combustion chamberengine, where each piston will be in a different stroke position, thevalve may respond to low pressure developed in the exhaust system whenone or all of the pistons are at other stroke positions.

[0013] It will be appreciated that reference in this specification toprimary air being induced into the or each combustion chambers includesair being introduced into the combustion chamber or chambers by e.g.turbo or other primary air introduction means.

[0014] The aspirator may be tuned to produce from the secondary air andcombustion products, which typically include un-burnt hydrocarbons,nitrous oxide gases (commonly known as “NOx”), carbon dioxide, carbonmonoxide and water (usually in the form of steam), reformed fuelincluding one or both of methanol and hydrogen gas, both of which maysatisfactorily provide a supplementary fuel. By operating an engine inaccordance with the invention, the amount of un-burnt hydrocarbons, NOxand carbon monoxide in the exhaust gases eventually emitted from theexhaust system to atmosphere has been found to be greatly reduced, andthe amount of carbon dioxide produced is significantly reduced

[0015] Preferably, the catalyst is provided by materials from which theaspirator is at least partly made, such as for example copper, but ifdesired Cerium may be provided in the vicinity of the aspirator which isparticularly good at enhancing secondary fuel production.

[0016] Where the apparatus is an engine, the reformed fuel may beintroduced into the or each combustion chamber of the engine as a resultof a partial vacuum being established at the exhaust port at the end ofan exhaust stroke of a piston thereof as the piston commences its nextinduction stroke, before the exhaust port is completely closed by anexhaust outlet valve or the piston.

[0017] Thus the reformed fuel will combust in the combustion chambersimultaneously with the primary air and primary fuel induced or injectedinto the engine during the next induction phase.

[0018] Where the combustion apparatus has a plurality of combustionchambers each having an exhaust port, the exhaust ports may each open toan exhaust manifold, where the aspirator or other means for introducingsecondary air into the exhaust system may be located.

[0019] The method of the invention may include introducing reformed fuelobtained by mechanically acting upon drawn in secondary air and productsof combustion in the exhaust system, into an inlet manifold from whereair is introduced into the combustion chamber for combustion with theprimary fuel.

[0020] This may be achieved by allowing reformed fuel introduced intothe combustion chamber via the exhaust port while the exhaust port andinlet ports are both not fully closed, to pass out of the combustionchamber into the inlet manifold for mixing with the air or air/fuelmixture in the inlet manifold, for subsequent introduction into thecombustion chamber together with primary and, and induced or injectedprimary fuel.

[0021] In addition to operating the apparatus in accordance with theprimary aspect of the invention described above, the method of theinvention may additionally include introducing into the combustionchamber of the apparatus, a vaporised secondary hydrocarbon based highcalorific value fuel, obtained by heating a mineral oil, for exampleusing an apparatus the subject of EP-A0744006.

[0022] A typical mineral oil may for example conform to British Standard245.

[0023] The mineral oil may be heated electrically and/or by heatexchange with hot exhaust gases produced by the engine.

[0024] It has been found that by introducing into the engine or othercombustion apparatus a vaporised secondary hydrocarbon based highcalorific value fuel to be burnt with the primary fuel and primary air,the engine may be operated at a lower temperature than otherwise, and asa result, the amount of particularly NOx produced during combustion, isminimised.

[0025] The combustion apparatus may be an internal combustion enginee.g. an engine in which combustion of fuel in the combustion chamber isinitiated by spark, or a Diesel cycle engine in which combustion isinitiated by heat caused by the compression of gases in the combustionchamber, or a spark assisted Diesel engine.

[0026] In the case of a spark ignition engine the primary fuel may beinduced and/or injected into the combustion chamber together with thevaporised secondary hydrocarbon based high calorific value fuel,although where the primary fuel is introduced by injection, thevaporised secondary hydrocarbon based high calorific value fuel may beintroduced with the induced air.

[0027] In each case performance of the invention has been found toimprove the power output of the engine, to achieve improved economy offuel use, and the reduction of harmful emissions.

[0028] According to a second aspect of the invention we provide a methodof adapting a combustion apparatus having at least one combustionchamber, for operation in accordance with the first aspect of theinvention, the method including providing the combustion apparatus withmeans to introduce into the exhaust system secondary air, providingmeans mechanically to act upon the secondary air and products ofcombustion in the exhaust system in the presence of a catalyst, toproduce a reformed fuel, and means to introduce the secondary fuel intothe combustion chamber of the apparatus via the exhaust port forcombustion with primary fuel and primary air.

[0029] Thus the invention readily lends itself to retrofitting to anengine, whereas conventional catalytic converters for example areprovided to cleanse exhaust emissions have to be provided as originalequipment e.g. by a vehicle manufacturer to ensure that there is spaceprovided to accommodate the converter and to ensure that the converterand engine operate satisfactorily together. However a vehicle may beadapted to perform the invention during manufacture as desired.

[0030] Where the engine to be adapted to perform the invention includesa catalytic converter already, depending on the kind of engine controlemployed, steps may need to be taken to ensure satisfactory operation ofthe engine. For example where the engine control is so called“open-loop”, i.e. an engine management system of the vehicle is operatedsubstantially independently of the performance of the catalyticconverter, the method of the second aspect of the invention may beperformed without any additional steps being required. However where theengine control is so called “closed-loop” in that an input to the enginemanagement system is provided from a sensor associated with thecatalytic converter, the method may include disabling the sensor of thecatalytic converter or re-programming the engine management system sothat performance of the method of the first aspect of the invention doesnot detrimentally affect the performance of the engine managementsystem.

[0031] It will be appreciated that catalytic converters operate mostsatisfactorily with a rich fuel/air mixture being burnt in the engine.With a so called “closed-loop” engine management system, the sensorassociated with the catalytic converter senses the amount of un-burntfuel and carbon monoxide, in the combustion products, and the enginemanagement system sets the richness of the primary fuel/air mixture foroptimum performance of the catalytic converter. Because utilising thepresent invention un-burnt hydrocarbons and carbon monoxide are used upin providing reformed fuel, an un-modified closed-loop engine managementsystem would respond to the sensor unduly to increase the richness ofthe primary fuel/air mixture.

[0032] The invention will now be described with reference to theaccompanying drawings in which:

[0033]FIG. 1 is a schematic illustration of an engine being operated inaccordance with the first and second aspects of the invention;

[0034]FIG. 2 is an illustrative partly exploded side sectional view ofan exhaust aspirator for use in the method of the invention;

[0035]FIG. 3 is an illustrative side sectional view of a secondary fuelapparatus for use in the method of the invention.

[0036] Referring to FIG. 1, an engine 10 has at least one combustionchamber 11. In this example the engine 10 is of the reciprocating type,in which the combustion chamber 11 is a cylinder in which a piston 12reciprocates. The invention is however applicable to other kinds ofinternal combustion engines such as for example rotary engines.

[0037] The or each combustion chamber 11 has an inlet port 15 throughwhich, in this example, primary combustion air is introduced, either bysimple induction due to piston 12 movement, or with turbo assistance,and an exhaust outlet port 16. Both the inlet and outlet ports 15, 16 inthis example are opened and closed by the operation of respective valves17, 18 which typically are controlled to open and close in accordancewith the engine cycle, e.g. by operating rods driven from a camshaft.

[0038] In this example, the engine is a Diesel engine in which primaryfuel for combustion is injected into the engine by an injector 19, andis ignited as a result of heat generated as air is compressed in thecylinder. In another example, ignition of the primary fuel may beachieved with spark assistance, and particularly in the case of a petrolspark ignition engine, primary fuel may be introduced into thecombustion chamber 11 along with the primary air.

[0039] The primary air may be introduced into the combustion chamberfrom an inlet manifold 20. Of course where the engine 10 has a pluralityof combustion chambers 11, the inlet ports 15 for each may be connectedto the inlet manifold 20. Air is provided to the inlet manifold 20 froman air inlet 21, via air filters etc. as is well known in the art,and/or the air may be provided via a turbo device or otherwise.

[0040] The or each exhaust outlet port 16 is connected to an exhaustmanifold 23 of an exhaust system 24. Again in the case of amulti-combustion chamber 11 engine, the outlet ports 16 for eachcombustion chamber 11 may be connected to the exhaust manifold 23.Combustion gases pass, possibly via a catalytic converter 25, from theexhaust manifold, to atmosphere.

[0041] In accordance with the invention there is provided a means forintroducing secondary air into the exhaust system 24, and in the presentcase, this is an aspirator device 26 which is described in more detailbelow with reference to FIG. 2, the aspirator 26 being adapted also toprovide for exhaust gases and introduced secondary air in the exhaustsystem 24, mechanically to be acted upon such that the exhaust gases andsecondary air provide a reformed fuel. The reformed fuel is then fedback into the combustion chamber 11 as hereinafter explained, and issubsequently burnt in the combustion chamber 11 along with primary fuel.

[0042] It will be appreciated that immediately subsequent to the piston12 performing an exhaust stroke in the combustion chamber 11, i.e.moving upwardly as seen in the drawings whilst the exhaust outlet port16 is open, and the piston 12 moves downwardly again to enable at leastprimary air to be introduced into the combustion chamber 11 with theinlet port 15 open, there will be a short but finite period in whichboth the inlet 15 and outlet 16 ports will simultaneously be open.During that period, a reduced pressure, or partial vacuum will beestablished at the outlet port 16 and as a consequence, gases from theexhaust system 24 will be drawn into the combustion chamber 11 alongwith the primary air from the inlet 15 port. Thus where the exhaustgases include reformed fuel, the reformed fuel will be introduced intothe combustion chamber 11.

[0043] Moreover, a very small amount of the reformed fuel may passthrough the combustion chamber 11 to the inlet port 15, andnotwithstanding flow will be predominantly in an opposite direction, avery small amount of the reformed fuel may pass to the inlet manifold 20for mixing with primary air. Thus the primary air subsequentlyintroduced into the combustion chamber 11 may contain a very smallproportion of reformed fuel.

[0044] Moreover, in FIG. 1 there is shown an apparatus 30 by means ofwhich a secondary high calorific value vaporised fuel may be introducedinto the inlet manifold 20, for introduction with the primary air intothe combustion chamber.

[0045] Referring now to FIG. 2, an aspirator 26 suitable for allowingsecondary air to be introduced into the exhaust system 24 is shown. Theaspirator 26 is tuned to maximise reformed fuel production from thesecondary air and combustion products.

[0046] The aspirator 26 includes a tube 32 which may be provided at oneend 33, i.e. the lower end indicated in FIG. 2, with a male threadedboss, to enable the aspirator 26 conveniently to be attached to theoutlet manifold 23, but the manifold 23 and aspirator 26 may otherwisebe arranged to be connected.

[0047] The tube is preferably made of a catalytic material, such ascopper, which promotes reformed fuel production, and if desired, Ceriumor another higher value but more efficient catalytic material, may bepresent in the exhaust system 24 further to promote the production ofreformed fuel.

[0048] At an opposite end 34 of the tube 32, at the top as shown in FIG.2, there is provided a valve 35. The valve 35 includes a valve seat 36accommodated in a housing 37 which is in the form of a mushroom shapedcap in this example. A valve member, which in this case is a ball 40, isurged by a spring 41 into engagement with the valve seat 36 which isshaped to receive the ball 40, normally to close the valve and preventair passing the valve 35 into the tube 32. However the spring 41 isweak, and in the event that a low pressure or partial vacuum isexperienced within the tube 32, the force of the spring 41 will beovercome and the ball 40 will be removed from the valve seat 36 so thatair may be drawn in through the valve 35, and hence to tube 32 and theexhaust system 24.

[0049] The aspirator 26 may be tuned to maximise reformed fuelproduction by setting the spring 41 pressure so that the valve 35 opensto maximise secondary air inflow at appropriate points in the enginecycle. Moreover, the spring 41 and valve 35 construction generally areadapted so that the secondary air is introduced as strong pulses, withthe valve opening and closing sharply. The pulses thus produced,mechanically act upon the combustion products and secondary air in theexhaust system 24 in the presence of the catalytic material, to reformfuel, typically hydrogen gas and methanol, which are introduced asdescribed above into the combustion chamber 11.

[0050] Other aspirator 26 constructions are no doubt possible. Forexample the valve 35 construction may incorporate other than a ballshaped valve member 40, e.g. a disc shaped valve member, with the valveseat 36 being appropriately configured so that the aspirator 16 is tunedto maximise reformed fuel production. In FIG. 1, the tube 32 is shown tobe bent, whereas in FIG. 2, the tube 32 is straight. The tube 32 may bebent as desired to fit within an engine compartment of a vehicle.

[0051] In a practical embodiment, typically, the tube is about thirtycentimeters in length, and typically the valve 35 is about 5 cm inlength. The tube 32 diameter is typically about 0.79 cm.

[0052] The operation of the aspirator 16 has been described withreference to particularly a single combustion chamber 11 engine 10, inwhich the low pressure or partial vacuum necessary to cause the valve 35to open to allow secondary air to be drawn into the exhaust system 24,occurs when the piston 12 is at or substantially at the top of itsstroke with the inlet 15 and exhaust ports 16 open. It will beappreciated that in a multi-combustion chamber 11 engine 10, in whichall of the pistons 12 will be at different positions in their cycles atany one time, in order to balance the engine 10, the valve 35 of theaspirator 26 may be arranged to open at a time which may not coincidewith the piston 12 of any combustion chamber 11 being at the top of itsstroke with the inlet 15 and exhaust ports 16 open. In amulti-combustion chamber 11 engine 10, the pressure in the exhaustsystem 24 will tend to change according to a complex pattern.Nevertheless, the valve 35 of the aspirator 26 may be arranged to openagainst the force of spring 41 when a set low pressure or partial vacuumis experienced in the exhaust system 24. Thus secondary air/reformedfuel may dwell in the exhaust system 24 but be introduced into thecombustion chambers 11 when next a respective piston 12 is at the top ofits stroke with the inlet 15 and exhaust ports 16 open. Thus the lowpressure or partial vacuum required for the introduction of secondaryair into the exhaust system 24 may not coincide with the low pressure orpartial vacuum experienced locally of the exhaust outlet ports 16 of thecombustion chambers 11 necessary for the reformed fuel to be introducedinto the combustion chambers 11.

[0053] Referring now to FIG. 3, there is shown an example of anapparatus 30 by means of which a secondary hydrocarbon based highcalorific value vaporised fuel may be introduced into the combustionchamber 11 (or chamber) along with primary air for combustion.

[0054] The apparatus 30 includes a reservoir 50 in which there isprovided a hydrocarbon based liquid fuel, which conforms to BritishStandard 245. The container 50 may be filled with liquid fuel via afiller cap 51, and a dipstick 52 may be provided so that the liquid fuellevel within the container 50 may readily be determined.

[0055] Mounted in the reservoir 50, is an elongate inwardly extendingelectrical heater element 55, which is mounted via a threaded boss 56.The boss 56 is in threaded and sealing engagement with the container 50,or at least of a bottom wall 57 thereof in this example, and alsoprovides a mounting 58 for a sub-assembly 59. The sub-assembly 59includes a cylindrical housing 60 in which there is provided a coiledtube 61 which opens through the housing 60 at one end 62 thereof, into aspace 65 above the level of the liquid fuel in the container 50, and ata lower end 64, opens through the housing 60 below the liquid fuellevel.

[0056] When the electrical element 55 is actuated, by passing anelectrical current therethrough, liquid fuel in the coiled tube 61 isheated and vaporised. The vaporised fuel passes into the space 65 abovethe liquid fuel level, and hence through an outlet 66 to the inletmanifold 20 of the engine 10. Thus the vaporised fuel passes with theprimary air, into the combustion chamber 11.

[0057] Around the coiled tube 61 in the housing 60, there is insulation63 so that only the liquid fuel in the coiled tube 61 becomes heated.Moreover because the coiled tube 61 is necessarily of small diameter andthus liable to blockage, the sub-assembly 59 is removable form thecontainer 50 by unscrewing from the mounting 58. To seal thesub-assembly 59 with respect to the container 50, the sub-assembly mayin use, be also in threaded engagement with the container via asecondary mounting 68, and may include some means, such as a knurledring 70, to facilitate removal and insertion of the sub-assembly 59 inthe container 50.

[0058] In another construction of secondary fuel apparatus 50, insteadof an electrical heater 55, or in addition thereto, means may beprovided to enable the liquid fuel to be heated towards vaporisation, byanother heat source, such as for example, hot exhaust gases produced bythe engine 10.

[0059] It will be appreciated that an existing engine 10 may be adaptedto perform the invention by providing an aspirator 26 tuned to reformedfuel production, in the exhaust manifold 23, and if desired by providinga secondary fuel providing apparatus such as apparatus 50 described towork in conjunction therewith.

[0060] Where the exhaust system 24 includes a catalytic converter 25,depending on whether this is a passive or active device, when adapting aengine for performance of the present invention, additional steps mayneed to be performed.

[0061] For example, where the catalytic converter 25 includes a sensor80 which provides an input to an engine management system 81, indicativeof the concentration of un-burnt hydrocarbons and carbon monoxide in theexhaust gases, the engine management system conventionally responds bysetting the richness of the primary fuel/air mixture to a level wherethe catalytic converter 25 can operate most efficiently. Where theinvention is performed, and the level of un-burnt hydrocarbons andcarbon monoxide in the exhaust gases are reduced by the method, thesensor 80 needs to be disabled and/or the engine management system 81re-programmed to ensure that the richness of the primary fuel/airmixture is not unduly increased.

[0062] In an engine in which for example the secondary fuel apparatus 50is only operated at intervals, the sensor 80 may be used, or anothersensor provided, to provide an input to the engine management system 81in response to particular engine conditions arising to cause thesecondary fuel apparatus 50 to operate or not to operate, e.g. byswitching the electrical heating element 55 or other heating means on oroff.

[0063] Various modifications may be made to the apparatus describedwhilst enabling the methods of the invention to be performed.

[0064] For example, in the example described, reformed secondary fuel isintroduced into the combination chamber via the exhaust port, forcombustion with the primary fuel. Particularly where an excess ofreformed fuel is produced, this may be collected and stored, andintroduced into the combustion chamber later, e.g. along with theprimary fuel rather than via the exhaust port.

[0065] The invention is not exclusively applicable to internalcombustion engines, but may be applied to other combustion apparatussuch as a boiler or furnace having a combustion chamber preferably withan exhaust port through which reformed fuel may be introduced

1. A method of operating a combustion apparatus (10) which includes atleast one combustion chamber (11) with an inlet port (15) for primarycombustion air, means (19) to introduce into the combustion chamberprimary fuel for combustion with the primary air, an exhaust port (16)for combustion products, and an exhaust system (24) for exhausting thecombustion products to atmosphere, the method including introducing intothe exhaust system (24) secondary air, mechanically acting upon thesecondary air and products of combustion in the exhaust system (24) inthe presence of a catalyst, to produce a reformed fuel, introducing thereformed fuel into the combustion chamber (11) for combustion withprimary fuel and primary air.
 2. A method according to claim 1characterised in that the reformed fuel is introduced in the combustionchamber (11) via the exhaust port (16).
 3. A method according to claim 1or claim 2 characterised in that the combustion apparatus (10) is aninternal combustion engine and secondary air is introduced into theexhaust system (24) via an exhaust aspirator (26) which draws air intothe exhaust system (24) during low pressure or partial vacuum conditionsoccurring during the cycle of pressure changes which occur in theexhaust system (34) during normal operation of the engine (10).
 4. Amethod according to claim 3 characterised in that the exhaust aspirator(26) is tuned to draw air into the exhaust system (24) and mechanicallyto act upon the secondary air and products of combustion by means ofpressure pulses, in a manner to optimise reformed fuel production.
 5. Amethod according to claim 3 or claim 4 characterised in that theaspirator (26) includes a valve (35) with a spring biased valve member(40), the force of the spring (41) acting to urge the valve member (40)into engagement with a valve seat (36) to close the valve (35), and theforce of the spring (41) being overcome when sufficiently low pressureis developed in the exhaust system (24), to allow the secondary air tobe drawn in through the valve (35) and the aspirator (26) being tuned byadjusting the spring (41) pressure so that the valve (35) responds to adesired low pressure or partial vacuum at an appropriate time during theengine cycle, and in a manner to produce strong pressure pulses in theexhaust system (24) adequate mechanically to act upon the combustionproducts and secondary air present in the exhaust system (24) in thepresence of the catalyst, to produce reformed fuel.
 6. A methodaccording to any one of the preceding claims characterised in that thereformed fuel includes one or both of methanol and hydrogen gas.
 7. Amethod according to any one of claims 3 to 5 characterised in that thecatalyst is provided by materials from which the aspirator (26) is atleast partly made.
 8. A method according to any one of the precedingclaims characterised in that the combustion apparatus is a reciprocatingpiston engine (10) and the reformed fuel is introduced into the or eachcombustion chamber (11) of the engine (11) as a result of a partialvacuum being established at the exhaust port (16) at the end of anexhaust stroke of a piston thereof as the piston (12) commences its nextinduction stroke, before the exhaust port (16) is completely closed byan exhaust outlet valve (18) or the piston (12).
 9. A method accordingto any one of the preceding claims characterised in that the combustionapparatus (10) has a plurality of combustion chambers (11) each havingan exhaust port (16), the exhaust ports each opening to an exhaustmanifold (23), where the means for introducing secondary air into theexhaust system (24) is located.
 10. A method according to any one of thepreceding claims characterised in that the method includes introducingreformed fuel obtained by mechanically acting upon drawn in secondaryair and products of combustion in the exhaust system (24), into an inletmanifold (20) from where air is introduced into the combustion chamber(11) for combustion with the primary fuel.
 11. A method according to anyone of the preceding claims characterised in that the method of theinvention includes introducing into the combustion chamber (11) of theapparatus (10), a vaporised secondary hydrocarbon based high calorificvalue fuel, obtained by heating a mineral oil electrically and/or byheat exchange with hot exhaust gases produced by the apparatus (10) tovaporise the oil.
 12. A method according to claim 11 characterised inthat the combustion apparatus 10 is a spark ignition engine and theprimary fuel is induced and/or injected into the combustion chamber (11)together with the vaporised secondary hydrocarbon based high calorificvalue fuel.
 13. A method according to claim 11 characterised in that thecombustion apparatus (10) is an engine is of the kind in which theprimary fuel is introduced by injection, and vaporised secondaryhydrocarbon based high calorific value fuel is introduced with theinduced air.
 14. A method of adapting a combustion apparatus (10) havingat least one combustion chamber (11), for operation in accordance withany one of the preceding claims, the method including providing thecombustion apparatus (10) with means to introduce into the exhaustsystem (24) secondary air, providing means (26) mechanically to act uponthe secondary air and products of combustion in the exhaust system (24)in the presence of a catalyst, to produce a reformed fuel, and means tointroduce the secondary fuel into the combustion chamber (11) of theapparatus (10) via the exhaust port (16) for combustion with primaryfuel and primary air.
 15. A method according to claim 14 characterisedin that the combustion apparatus (10) is an engine of a vehicle, thevehicle engine (10) of which is to be adapted includes a catalyticconverter (25) already and an engine management system (81) thereof isprovided with an input from a sensor (30) associated with the catalyticconverter (25) which affects engine control, the method includingdisabling the sensor (80) of the catalytic converter or re-programmingthe engine management system (81).